The manufacture of printed circuit boards and the process of plating metal on plastic substrates involves an electrolytic or electrodeless deposition of a type of heavy metal, most commonly copper, nickel or chromium onto an appropriately prepared surface. Many of the processes used in this manufacturing process involve the use of strong complexing or chelating agents such as ethylene diamine tetra acetic acid, known as EDTA, nitrilotriacetic acid, known as NTA, tartrate, Quadrol, citrate, glycolate, ethanolamines and ammonia.
For example, a publication by R. E. Wing and W. E. Rayford, entitled, "Heavy Metal Removal Using Dithiocarbamates", Plating and Surface Finishing, 69(1), 1982, pp 67-71, describes how the dithiocarbamates are utilizable to remove heavy metals. U.S. Pat. No. 4,731,187 issued to Moriya et al, entitled "Removal of Heavy Metals From Waste Water" discloses the use of metal scavengers containing dithiocarbamic acid compounds. The Moriya technique involves the use of a metal scavenger containing at least one carbodithio group and/or at least one carbodithioate salt group as N-substituents per molecule, together with at least one of sodium monosulfide, sodium polysulfides and sodium hydrogen sulfide to waste water containing heavy metal ions so as to scavenge and remove the metal ions from the waste water. The practice of this invention creates a heavy metal bearing sludge which has to be disposed.
U.S. Pat. No. 4,578,195, issued to Moore et al, entitled "Process for the Purification of Effluents and Purge Streams Containing Trace Elements" discloses the method of contacting the heavy metal effluent with a poly(dithiocarbamate) chelating agent whereby the polluting elements are absorbed. The patent states that after contact with the poly( dithiocarbamate) resin, the residual content of the polluting elements in the effluent is reduced to a level below the environmental Protection Agency required levels so that the effluent can safely be discharged int.degree. the surrounding ecosystem.
The Moore patent goes on to state that the poly(dithiocarbamate) resin employed in the Moore process rigidly retains the chelated elements and that the poly(dithiocarbamate) resin containing the chelated elements can be disposed of in a designated sanitary landfill area. Disposal of heavy metal bearing sludge produced by the method disclosed in the Moore patent represents a cost of disposal as well as the loss of the value of the metals contained therein.
Complexing agents, including those described above, form water soluble metal complexes critical for the proper deposition of a uniform, smooth metallic surface. However, the soluble metal complexes also render the metal difficult to remove by conventional waste water treatment techniques such as precipitation by pH adjustment or by addition of ferrous sulfate.
Alkali metal salts of dithiocarbamic acids, and most commonly sodium or potassium salts of N,N-diethyl- or N,N-dimethyldithiocarbamic acids, have been found to effectively precipitate heavy metals from waste water containing complexing or chelating agents. The dithiocarbamate method of heavy metal ion removal from waste water effectively removes the heavy metal, but produces large quantities of heavy metal bearing sludge which must be disposed of in an approved hazardous landfill. Consequently, sludge disposal is very expensive, environmentally detrimental, wasteful of natural resources, and involves compliance with local, state and federal regulations. In addition, the sludge producer often retains long term liability for the sludge despite the manner of its disposal/waste storage.